The present invention relates to a fixing apparatus with heat and pressure for electrophotographic copiers.
In accordance with a general process employed in electrophotographic copiers, a toner image is electrostatically formed on a photosensitive member, and said toner image is transferred to a transfer paper, after that, it is fixed. Fixing apparatus for fixing the toner image heretofore employed is a fixing apparatus with heat and pressure.
This apparatus has a structure such that a heating roll having a surface coated with a heat-resistant releasing material and a pressure roll having a heat resistance and a flexibility are brought into contact with each other by applying a given pressure to permit a paper to be fixed to pass therebetween.
Heat resistant releasing materials for the heating roll which have been known are tetrafluoroethylene (Teflon), silicone rubber, fluorocarbon resin, etc., and as the case may be, fixing is carried out while further forming a film such as a suitable silicone oil.
The heating roll coated with silicone rubber among these heat resistant materials is resilient in its surface so that smash or deformation of the toner may be minimized to obtain copies of good quality.
Such silicone rubber is roughly divided into at least two kinds from its vulcanization temperature. That is, there are room temperature vulcanized silicone rubber (RTV) and high temperature vulcanized silicone rubber (HTV), the RTV being generally better than the other in terms of the parting property relative to the toner. In the case of rolls currently used, the good parting property remains exhibited in the range of copies from about 5,000 to 30,000, but when the silicone components run dry, then unfavorable situations such as twining of paper round the offset or roll result.
On the other hand, application of silicone oil to the surface of the heating roll provides a significant effect in preventing offset, but both RTV and HTV are swelled by the presence of silicone oil to unfavorably induce deterioration of various properties and poor adhesion.
A method for heating a heating roll is also roughly divided into two methods, one of which is to heat the heating roll from the interior thereof and the other receiving heat from the external surface thereof.
In the former method, it is not complex in construction, but heat transmission inherent in the silicone rubber is so bad as to increase a temperature of adhesive interface with a core of the heating roll thereby inducing thermal deterioration of primer and deterioration of various properties. For this reason, a method has been proposed wherein rubber is formed into a multi-layer, into which thermally stabilized rubber is inserted. This method, however, results in an increase of manufacturing process and an increase of rubber wall from a thermal standpoint, which is not effective.
In the latter method, the external surface is highest in temperature so that the disadvantages noted above may be decreased. However, it is important how to enhance transmission efficiency to the external surface and it is also important how to minimize heat loss caused by an increase of the entire surface area. In addition, where an infrared heating source is used as the external heating source, there is a possible danger of fire caused by the paper when the latter is plugged or twined.
Further, most of such heat resistant releasing materials are small in heat conductivity, and hence, as compared to a heating roll which uses a metal surface without modification with respect to thermal transmission rate to the paper to be fixed, the temperature rise is slow internally of the transfer paper with which the heating roll comes into contact, as a consequence of which more heat is required to let it increase to the same surface temperature. However, the surface energy of the metal surface is extremely high and lends itself to what is called the offset phenomenon in which a part of toner image is adhered to the heating roll surface, and even if a film such as silicone oil is formed on the surface, the toner passes through the film to permit the toner to touch with the metal surface to induce offset, which is not practical.
In order to decrease the offset, there has been proposed a method for heating two rolls so as to provide a temperature gradient within the toner layer, that is, to semi-dissolve the toner surface while to dissolve the paper to be fixed. However, since the temperature conductivity of the paper to be fixed is extremely low, the roll on the side of the paper to be fixed must be held at a relatively high temperature, resulting in a poor thermal efficiency.
Further, for purposes of minimizing the aforesaid offset, all that need be done is at least not to allow the toner to come into contact with the surface of the heating roll. As for example, one method has been proposed, in which a functional group is introduced into a releasing agent such as silicone oil to effect chemical reaction with the surface of the metal roll thereby forming a cross link film of which low surface energy is formed to minimize offset. The functional group specifically used is a polyalkylsiloxane releasing agent containing mercapto group, carboxyl, hydroxyl group, epoxy group, amino group, isocyanate group, and thioether, and metals well-used are iron, copper, aluminum, zinc titanium, silver, etc. In this case, thermal transmission efficiency may be increased to decrease electric power required, while the surface is formed with a hard metal to smash the toner conversely to the resilient surface such as silicone, resulting in the lowering of image quality such as an increase of background density of the image.